This invention relates to new compositions for manufacturing glass with high refractive index of over 2.10, especially suited for producing glass beads to be used in reflex-reflective sheet for road traffic signs, vehicle number plates etc., and also can be used as glass flakes and fibers etc.
The glass for producing glass beads to be used for reflex-reflection, must be not only highly resistant to weathering, chemical actions etc., but the glass beads must be essentially clear and transparent without devitrification, and be very minute of less than 0.1 mm in diameter.
Heretofore, there have been various compositions suggested for this type of glass, but generally in order to obtain glass with high refractive index, it was considered that a high content ratio of TiO.sub.2, PbO etc. would be effective.
For example, it is disclosed in U.S. Pat. No. 2,790,723 that refractive index of at least 2.10 can be obtained from TiO.sub.2 - PbO system glass, but 10 to 65% content of PbO was an indispensable requirement for said composition.
However, the use of PbO as an ingredient in glass composition, not only produces a yellowish color to the glass, but also causes harmful effects to the workers, so the general opinion is to avoid the use of PbO and produce high refractive index glass by a composition that does not contain PbO.
Therefore, endeavors have been made to fulfil the initial aim of avoiding the use of PbO by increasing the TiO.sub.2 content.
Namely, U.S. Pat. No. 2,726,161, U.S. Pat. No. 2,870,030 etc. have suggested TiO.sub.2 - Bi.sub.2 O.sub.3 system glass wherein all or part of the PbO is replaced by a high content of Bi.sub.2 O.sub.3. Nevertheless, in the former example, 0 to 55% by weight of PbO content is required in its compositions, and since most of the example compositions shown contain PbO, the aforementioned problem has been left unsolved. Even in the latter patent, in which large amounts of Bi.sub.2 O.sub.3 have been included, there are drawbacks due to the fact that high content of Bi.sub.2 O.sub.3 causes yellow or brown coloring to glass and also the fact that the raw material for Bi.sub.2 O.sub.3 is very expensive.
Accordingly, in order to obtain compositions for producing glass with refractive index of over 2.10, glass, such as disclosed in U.S. Pat. No. 3,493,403, of TiO.sub.2 - RO system having a high content of over 55% by weight of TiO.sub.2 and glass formed by the addition of other metal oxides to such system were suggested. It had been considered that the formation of such a glass was very difficult as the composition had a high fusion temperature and moreover the glass became readily devitrified. However the foregoing patents disclosed that even in the case of such a composition, it was possible to produce glass with refrective index of at least over 2.10 by a method differing from the past of using a very high temperature of over 2,700.degree. C. that greatly exceeds the temperature used for conventional glass industry, and by rapid forced quenching.
However, it goes without saying that the use of such extremely high temperature would pose many difficult technical problems, so such a method should be avoided as much as possible. Moreover, since TiO.sub.2 is an ingredient that readily causes devitrification it is deemed that very large TiO.sub.2 content should be avoided as practically as possible, providing the aforementioned refractive index of over 2.10 can be obtained.
Consequently, the object of this invention is to produce glass with refractive index of over 2.10, suited for forming minute glass beads of less than 0.1 mm in diameter and for making glass flakes and fibers etc. that can be readily formed by the conventional glass making method, without using the harmful PbO and CdO and the aforementioned Bi.sub.2 O.sub.3 and thereby obtain a composition for a stable glass that would not devitrify during glass fomation. In order to fulfil the object of this invention, repeated tests and experiments were conducted of various compositions using the conventional method of melting at about 1,400.degree. C. within a crucible and small tank furnace.
From the results of actual tests, it was discovered that essentially colorless and transparent glass having a refractive index of over 2.10 could be produced by said conventional method without using PbO or Bi.sub.2 O.sub.3 ingredients, essentially from the compositions containing the following ingredients in weight percent; 40.0 to 54.0% of TiO.sub.2, 24.0 to 44.0% of BaO, 5.0 to 15.0% of ZnO, 3.0 to 13.0% of ZrO.sub.2 as main constituents together with 0.2 to 3.0% of CaO, 0.2 to 3.0% of MgO, 0.2 to 2.0% of SiO.sub.2, 0.1 to 0.5% of Na.sub.2 O and/or K.sub.2 O, provided that the total weight percent of said ingredients used within the specified limits would exceed over 98%.